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12.3 Origin of Life KEY CONCEPT The origin of life on Earth remains a puzzle. 12.3 Origin of Life Earth was very different billions of years ago. • There have been many hypotheses of Earth’s origins. • The most widely accepted hypothesis of Earth’s origins is the nebula hypothesis. 12.3 Origin of Life Several sets of hypotheses propose how life began on Earth. • There are two organic molecule hypotheses. – Miller-Urey experiment electrodes “atmosphere” water “ocean” heat source – meteorite hypothesis amino acids 12.3 Origin of Life • There are different hypotheses of early cell structure. – iron-sulfide bubbles hypothesis 12.3 Origin of Life • There are different hypotheses of early cell structure. – lipid membrane hypothesis 12.3 Origin of Life • A hypothesis proposes that RNA was the first genetic material. – Ribozymes are RNA molecules that catalyze their own replication. – DNA needs enzymes to replicate itself.